constantly?
My daughter is always touching herself now. She sits spread eagle and just goes to town/and or does it casually while playing. I know its probably normal, and it doesn't really make us uncomfortable (it did just start sort of bugging me for no real reason). We decided she's still to young to have the "you do that in private" conversation and we just try to keep her clothed when we have guests.
Tell me it's normal/healthy and will not last forever!
She gets plenty of time to "explore", and is almost always naked waist down. In fact, we're working on getting her to wear clothes more because it's getting colder.

Really, I'm happy to see her exploring her sexuality, since I have not had the healthiest attitude about it in my life.

My DS is often touching/enjoying his penis. I do not, however, term it masterbating. For me, masterbating is touching with sexual intent, ie, to orgasm. This does not fit with a small child. They are exploring. They will also explore picking their noses. Same deal.

Now, we're potty learning and are also almost always nakey-bum. And he grabs himself right before he pees. So if him touching himself is bothering me, I'll ask him "Do you need to pee? No? Oh, when I saw you holding your penis I thought maybe you needed to pee." At that, he usually lets go of it for a little while. Now, I have no idea if this is considered correct or not, but I figure it's better than guilting him or freaking out on him. We have tried underwear, but he still pees in them.

As for wearing more clothes as it gets colder, I just offer him clothes more often. "Do you want to wear a shirt or go naked?" He can choose for himself when he's cold enough to put more clothes on!

Does this ever go away? Um, good question! It's considered a self-soothing thing. Maybe more one on one time with y'all would decrease the frequency? More likely she's just found a new toy that goes with her everywhere! The fascination with any new toy has to die down eventually, right?

My dd does this (same age as yours)- I don't think I'd call it masturbation, yet, though - it looks to me like absent-minded toying. And it's opportunistic - she does it when she's naked, but doesn't seek to get into her clothes to do it. We just ignore it, like you've been doing, and at this stage of the game I think they would be highly confused by the privacy talk. It might even accidentally shame them, which is why we haven't done it and don't plan to unless it becomes a problem when she's older. I think it's normal at this age - you probably don't have anything to worry about!

DS does this too! He's also started taking his diaper off so he can have access (he's 15 mo.). It's actually hilarious to try to see him walk (a recent milestone) while pulling on it. Hasn't quite got that level of multitasking down yet